Cracks that form within materials can be difficult to detect and almost impossible to repair. A successful method of autonomically repairing cracks that has the potential for significantly increasing the longevity of materials has been described, for example, in U.S. Pat. No. 6,518,330. This self-healing system includes a material containing, for example, solid particles of Grubbs catalyst and capsules containing liquid dicyclopentadiene (DCPD) embedded in an epoxy matrix. When a crack propagates through the material, it ruptures the microcapsules and releases DCPD into the crack plane. The DCPD then mixes with the Grubbs catalyst, undergoes Ring Opening Metathesis Polymerization (ROMP), and cures to provide structural continuity where the crack had been.
Crack formation in coatings can be especially problematic, since coatings are often present to protect the substrate onto which they have been coated. For example, metal substrates may be coated with a layer of material to prevent or inhibit corrosion of the metal. A crack in such a coating typically leads to corrosion of the underlying metal, resulting in expensive and wasteful repair or replacement of some or all of a part made from the metal. To ensure the integrity of the metal, it may be necessary to replace the coating periodically, regardless of whether cracks actually have formed. Coatings also may be used in the form of primers, paints, stains, sealers and topcoats. Substrates for these coatings include building materials, windows, electronics, automotive parts, marine parts and aerospace parts. These coatings may protect the underlying material from corrosion, moisture, bacterial growth, ultraviolet radiation and/or mechanical impact.
Self-healing materials typically are limited to autonomic repair of a single damage event in a given location. When self-healing is triggered by crack-induced rupture of embedded capsules, once a localized region is depleted of healing agent there can be no further repair. Re-mendable polymers can achieve multiple healing cycles, but typically require external intervention in the form of heat treatment and applied pressure. See, for example, Chen (2002) and Chen (2003).
It is desirable to provide a material that can self-heal when subjected to a crack multiple times in the same location. It is also desirable to provide a material that can function as a coating and that can self-heal to maintain the protection of the substrate from the surrounding environment.